This application is a 35 USC 371 application of PCT/DE 99/03643 filed on Nov. 16, 1999.
1. Field of the Invention
The invention directed to a piston pump and more particularly to a piston pump for supplying high pressure fuel to a fuel injection is an internal combustion engine.
2. Description of the Prior Art
One such piston pump is known (German patent disclosure De 42 13 798 A1) that has three pump elements which are each connected on the intake side to a low-pressure supply, via a spring-loaded check valve. The known piston pump is by its design a constant pump, whose pump elements are set to a maximum required volumetric flow in a fuel injection system. In quantity regulation of the fuel flow on the low-pressure side, however, the disadvantage arises that at a volumetric flow smaller than the maximum volumetric flow, unequal filling of the various pump elements ensues because of variations in the check valves on the intake side. The reason for this is that the intake-side check valve of the pump elements set for a small opening stroke is opened during the intake stroke of the pump piston and during part of the pumping stroke. Overlaps in the opening times of the intake valves of other pump elements can occur. However, since at a small volumetric flow the pressure in the low-pressure system is quite low and decreases further upon filling of a pump element, if the opening time of one intake valve is too long, the result can be incomplete or entirely absent filling of another pump element. In the high-pressure part of the fuel injection system, however, this causes pressure fluctuations, which adversely affect the operation of the internal combustion engine connected to it.
From British Patent GB 564 725, a piston pump with two intake-side check valves connected in series is known. The check valves, which are structurally identical, have a ball that is not spring-loaded as their closing member, which assumes its closing position by gravity. With the dual disposition of the intake-side check valve, the intent is to achieve improved tightness and effectiveness of the pump.
The piston pump of the invention has the advantage over the prior art that the partial filling of a given pump element no longer depends essentially on the cooperation among the feed pressure of the feed pump, the spring force of the first check valve, and the negative pressure generated by the pump piston; instead, the duration of filling of the pump element is determined by the second check valve, which is essentially loaded only by feed pressure and spring force, and the second check valve also limits the filling when the volumetric flow of the supplied fuel is low, while the first check valve in the compression phase of the pumping process now serves essentially only to block off the cylinder chamber of the pump element from the second check valve. The fuel metering operation is thus no longer determined by the duration of opening of the first check valve. The onset and end of the metering operation are initiated and defined according to the invention by the feed pressure of the fuel.